1. Field of the Invention
The present invention relates to a manufacturing process of a thermoelectric conversion element, in particular with respect to a thermoelectric conversion element having a nanostructure on a surface thereof.
2. Description of the Related Art
When facing the gradual exhaustion of the global primary energy as well as the more and more critical greenhouse effect, development along with application of renewable energy has become an issue in an urgent need. The technique of thermoelectric conversion, which converts thermal energy of the primary energy into a more valuable electric energy, does not have noises and byproducts in the process of the conversion as having no movable parts, and thus, it matches up the green energy of a concept of environmental protection.
More and more scholars have been engaging in the development of nanotechnology, enabling the related research on the thermoelectric conversion element becoming a doctrine gradually. However, the limitation of thermoelectric material degrades the conversion efficiency. Wherein the thermoelectric conversion element is standardized by the thermoelectric figure of merit
      ZT    =                            S          2                ⁢        T        ⁢                                  ⁢        σ            k        ,and S is Seebeck coefficient, T is absolute temperature, σ is electrical conductivity of the material, k is thermal conductivity. As the formula shows, increasing electrical conductivity of the material, or Seebeck coefficient S or decreasing thermal conductivity k all can enhance the thermoelectric figure of merit of elements so as to boost the thermoelectric conversion efficiency. The aforementioned parameters, however, are not individual, such that any one of them is not able to be fixed without affecting the remains, and thermoelectric figure of merit can't be boosted unlimitedly.
The thermal conductivity k and the electrical conductivity σ normally show a positive correlation change based on Wiedemann-Franz law
      LT    =          k      σ        ,wherein L is the Lorenz number of material, and when any of the thermal conductivity k or electrical conductivity σ is changed, the other corresponding parameters will change too, enabling that the thermoelectric figure of merit ZT doesn't have obvious modification. Consequently, metal material is less applied to the thermoelectric conversion element.
Compared with metal material, semiconductor material has better advantage of Seebeck coefficient S, and an order of magnitude difference of the Seebeck coefficient S is between the two materials. As semiconductor material is of lower thermal conductivity k, it is the commonly seen type used in the thermoelectric material. Because the electrical conductivity σ of semiconductor material is hard to be enhanced, most thermoelectric figure of merits of the existing thermoelectric conversion elements are less than 2, resulting that the technique has a long way to run in practical application.
In addition, regarding the prior art, it also uses semiconductor material as a substrate and then dopes a certain proportions or types of rare earth elements to adjust the effective electron concentration in the semiconductor material an as to enhance the thermoelectric figure of merit by boosting the electrical conductivity σ. But, by doping rare earth elements to change the electrical conductivity σ still has a technical bottleneck. For example, firstly, rare earth elements are not easy to obtain and the price thereof is expensive; secondly, the doping uniformity is not easy to be controlled; thirdly, the manufacturing process is too complicated, . . . and so on and so forth, leading the current thermoelectric conversion element to an unpopular way.
According to the preceding description, inventor of the present invention therefore designs a manufacturing process of a thermoelectric conversion element which aims to improve the shortcomings of the current technique so as to boost the industrial practicability.